This invention relates to fuel for nuclear reactors. Such nuclear reactors are discussed for example in "Nuclear Power Engineering", M. M. El-Wakil, McGraw-Hill Book Company, Inc., 1962.
Nuclear reactors are typically refueled periodically with an excess of fuel sufficient to maintain operation throughout an operating cycle.
This excess of fuel results in an excess of reactivity which requires a control system of sufficient strength to maintain the effective multiplication factor at unity during reactor operation. The control system customarily comprises neutron absorbing or poison materials that serve to control the neutron population by nonfission absorption or capture of neutrons. Typically, the control system includes mechanical control in the form of a plurality of selectively actuatable poison containing control rods or the like which can be inserted into and withdrawn from the core as required.
It is also known to include in the fuel core a burnable poison which is a strong neutron absorber but is converted by neutron absorption to an isotope of low control worth (neutron absorbing capacity). Such use of burnable poisons decreases the amount of mechanical control required and, by appropriate arrangement of the burnable poison, improvements in power distribution can be achieved thereby.
Such burnable poisons frequently are incorporated in the fuel core in a mixture with selected portions of the nuclear fuel. Such nuclear fuel is typically in the form of pellets or powder contained in an elongated cladding tube to form a fuel element as shown, for example, in U.S. Pat. No. 3,378,458. An arrangement of burnable poison in a fuel core is shown, for example, in U.S. Pat. No. 3,799,839.
It is desirable for quality control and identification purposes during nuclear fuel handling and manufacturing processes to have rapid nondestructive methods of determining the amount and location of the burnable poison in a nuclear fuel element. Since gadolinium is one of the most widely used burnable poisons, it is particularly desirable to determine the gadolinium content of nuclear fuel.
When an additive has a magnetic susceptibility sufficiently greater than the magnetic susceptibility of its admixed nuclear fuel, its susceptibility in a magnetic field can be measured to determine the location and amount of the additive in the fuel element.
Typical nuclear fuel, such as oxides of uranium and plutonium are paramagnetic. For example, the room temperature susceptibility of uranium dioxide (UO.sub.2) is 8.76.times.10.sup.-6 emu/g-Oe (electrmagnetic unit/gram-Oersted). Fortuitously, the room temperature susceptibility of gadolinia (Gd.sub.2 O.sub.3) is significantly greater, namely, 147.times.10.sup.-6 emu/g-Oe. This difference is sufficient to make practical the magnetic determination of gadolinium additive content in nuclear fuel.
A problem which complicates the magnetic determination of gadolinium or other paramagnetic additives in nuclear fuel is the presence of ferromagnetic impurities, such as iron, therein, typical fuel containing 100 ppm (parts per million) or more of such impurities. The susceptibility of such ferromagnetic impurities is very high in low magnetic fields but decreases greatly in high fields as they tend to become saturated.
Advantage is taken of this in a method and apparatus for magnetically determining the Gd.sub.2 O.sub.3 content in UO.sub.2 fuel pellets shown and described in U.S. patent application Ser. No. 754,581, filed 27 Dec. 1976, now U.S. Pat. No. 4,134,064. As therein described, the ferromagnetic inclusions are saturated with a high, constant magnetic bias field while the gadolinia content of the fuel is determined by measurement of the alternating current susceptibility using an inductive technique.
General information on magnetic materials and properties is given in Magnetism and Metallurgy, edited by A. Berkowitz and E. Kneller, Academic Press, New York, 1969.